Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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638
FXUS63 KMKX 190848
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
348 AM CDT Wed Jul 19 2017

.SHORT TERM...

TODAY AND TONIGHT - Confidence...Medium.

The overnight showers and storms have diminished and pushed off to
the east and south.  However weak frontal boundary remains over
southern WI along with some weak MUCape.  Hence can not rule out an
isolated shower or storm redeveloping next several hours, mainly
across southern CWA.  In addition, patchy fog may become briefly
dense due to the light winds and high low level moisture through the
early morning.  The fog threat will diminish by mid-morning as winds
pickup from the north to northeast and slightly drier air moves in
from the north. A mostly dry day lies ahead for the area with some
reprieve from the humid conditions in the northern CWA.  Humid
conditions wl linger across the southwest. Sunshine should help
temperatures recover into the 80s most area, but onshore flow wl
keep lakeshore temps a bit cooler in the mid 70s to around 80.

Stalled frontal boundary to the south of WI this afternoon will
begin moving back to the north tonight in response to strong
synoptic scale forcing moving across the upper midwest.  Right
entrance region of upper jet passes across the northern Plains into
the western Great Lakes tonight, resulting in a period of strong
upper level divergence moving across portions of MN into WI and IA
tngt.  All short term guidance in good agreement on vigorous low
level jet developing across southern MN this evening and then
spreading east or southeast overnight.  ECMWF and GEM spread strong
low level convergence mainly eastward into central and northern WI,
while GFS and NAM farther south.  Strong WNW winds at 700H and 500H
would typically carry the developing MCS southeast along the
strengthening quasi-stationary boundary that will extend from
southern MN across eastern IA into the vicinity of southern
WI/northern IL. Hence leaning more on southern solutions and wl
continue likely wording for thunder in the 06Z-12Z period.  Not
impossible a bow echo/high wind event may occur and affect the area.
SPC higher risk category may spread farther east into southern WI
later today. Expect tonights convection to move at a higher rate of
speed southeast, and not expecting storms to repeatedly affect an
area resulting in flooding rains.  However, some areas may still
receive an inch of rainfall overnight with high rainfall rates
resulting in ponding of water and minor street flooding.

.LONG TERM...

Thursday... Forecast confidence is Medium.

Showers and storms will taper off across southern WI Thu morning.
It will take several hours for the atmosphere to recover, and we
will likely lack forcing for additional showers or storms the rest
of the day since the front/outflow will presumably be down in IL.
Still, I kept the slight chance in the fcst in case any weak
boundaries can trigger a shower or storm.

We will still be in a warm and humid air mass, so expect highs to
reach the mid 80s but is dependent upon cloud cover. The front
will begin to lift back northward Thu night, so there are
increasing chances for storms after midnight.

Friday through Saturday... Forecast confidence is Medium.

Heavy rainfall possible this period. Southern WI is in a prime
area for showers and storms over a fairly long period of time late
this week. There will be zonal flow aloft with a few
ripples/shortwaves, the right entrance region of the upper jet
tracking over central WI, warm air advection, and moisture
transport on the nose of the low level jet from Friday afternoon
through Saturday morning. Precipitable water values are expected
to be around 2 inches during this time, which is around 175% of
normal. Showers with isolated thunder should be very efficient
during this time and we will continue to monitor this period for a
flash flood potential.

Saturday night looks dry as high pressure briefly passes by.

Sunday through Tuesday... Forecast confidence is medium.

A more amplified upper trough is expected to roll across Lake
Superior on Sunday. The associated surface low should move across
northern WI that drags a cold front through southern WI midday
Sunday. There could be a few showers and storms associated with
this, but moisture and instability are still questionable.

High pressure will keep us dry and relatively cool for the first
part of next week. Our next chance for storms will arrive with a
warm front Tuesday night.

&&

.AVIATION(06Z TAFS)...Light winds high low level moisture are
allowing patchy fog and low clouds to develop across the area. Would
expect this trend to continue for a few more hours until mid-morning
or so as winds become more northeasterly and increase.  Hence patchy
IFR cigs and vsbys may affect TAF sites for a few hours.  Otherwise,
a VFR day lies ahead.  Still looking like a period of thunderstorms
will affect parts of southern WI after midnight tonight.  Strong
winds may accompany the storms.

&&

.MARINE...

Recent MODIS imagery measured Lake Michigan surface temps in the 60s
to around 70.  Light winds for a time this morning along with high
surface moisture may result in areas of fog developing.  As drier
air moves southward later this morning and afternoon, the threat of
fog will diminish.  Dry conditions are expected most of today into
this evening as winds becoming north to northeast.  A few wind gusts
to 15-18 knots will be possible later this morning.  A period of
thunderstorms may sweep across the near shore waters after midnight
tonight, possibly producing strong wind gusts.

&&

.HYDROLOGY...

The round of thunderstorms tonight is expected to be progressive
with up to an inch of rain. Thus, this period does not look like a
flash flood threat or one that would exacerbate river flooding.

The time period to watch for excessive rainfall is Friday
afternoon through Saturday night. This active period of weather
has the potential for training of storms over the same areas to
bring the risk for heavy rainfall.

This would aggravate ongoing flooding across far southeast
Wisconsin, and create flooding issues elsewhere, especially in
urban areas. Once more confidence is gained with timing of these
rounds of storms, expected rainfall amounts will become more clear.
Keep up with the latest forecasts into this weekend.

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...None.
LM...None.
&&

$$

Today/Tonight and Aviation/Marine...MBK
Thursday through Tuesday...Cronce



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